We have the Jump for Fun Octojump and it's been brilliant. The blurb on the safety advantage of the octagonal vs round I have found to be very true, the kids don't end up in a puppy pile in the middle, alternatively go for a rectangular one. Go as big as you can manage to get the best out of any trampoline. Ours is like having another room in the summer, when they're worn out the they just lay around on it and chat!
It's a 14ft. We were going for the 12ft but at the time the 14 was on sale and cheaper than the 12 (probably less popular due to size... it is really huge!). I think whatever you can fit is good, but there are advantages to the larger sizes. Your children are still young so it depends also on how long you intend keeping it.
I really recommend digging a huge big hole and sinking your trampoline into the ground so that the jump surface is level with the ground. No need to lift toddlers and smaller children up and down while squeezing them through the safety net . Much easier on the back! It takes a bit of effort to get it level, but so worth it. Safer too for older kids as they won't fall from any height if the net is removed. I plan to build a decking cover disguise for ours too.
We bought ours from Atlantic Trampolines. My dad had bought one a few years ago from them and it is still in very good condition. He had to buy some parts as it blew over in the wind and they were very helpful and sent them next day. Ours is a 12ft one and we sunk it in the ground so did not need a net. I would recommend you get the largest one you can afford and will fit in your garden as it is surprising how often the children use it (and how many new friends they find!). It does pay I think to buy one which is good quality as the cheap ones only seem to last a Summer before they fall apart.